I have run the Davis UV sensor for some years, have done cleaning, and worked through some degradation concerns.

The first question is what comparison you are making that leads you to suppose the reading is too low. Cloudless day UV is surprisingly variable from year to year. So just comparing your reading this year to years past in a single snapshot is not good enough.

I don't know of any source around here which provides current UV readings from actual observation other than amateur postings from equipment like ours, which are quite variable. The national weather database that supports most other observations seems to limit itself to forecast numbers, and not actual current UV observations. One source that I have found reports the forecast from that source to 0.1 index number resolution, and allows specification to a specific lat/lon. It is WillyWeather.

I have compiled a spreadsheet with many days of observations of my Davis UV versus the WillyWeather forecast averaging an hour of readings at solar noon on cloudless days.

If others here can suggest a better way of "calibrating" one's UV reporting I am very interested to hear it.

Here in New Mexico, the perhaps half dozen times over the years that I have cleaned my solar and UV sensors, I have sometimes seen as much as a 5% increase in reported solar intensity, but I have not seen a clear change in reported UV. Attempting to respect the user manual requirement to use ethyl alcohol, I formerly employed Everclear, and recently have used some that I sourced on eBay from a Bulgarian vendor. But my types of sensor contamination may not match those at your location, so I think cleaning is worth a try.

About a year ago I believed that my UV sensor had lost sensitivity and as a temporary workaround (after cleaning failed to resolve the discrepancy) instituted a large calibration multiplier in Cumulus (40%--which was probably a bit too high).

About six months ago I resolved to buy a new sensor, figuring that I would then alternate sensors, sending each one in for refurbishment as needed, without having a gap in my reporting. But I learned that Davis in my case would carry out the refurbishment on a swap basis, sending out someone else's refurbished sensor for me to install, then crediting me when I returned mine, with the total cost considerably less than that of a new sensor. (they asked the manufacturing date of mine before telling me the deal, so it may not be the same in all cases).

So I did that. The replacement indeed reads higher than the previous one did in the degraded state it had reached, though with continued comparison to WillyWeather I have settled in on using a 1.15 multiplier.

I have two uv sensors running and they are .1 apart from each other but they weren't always that close. I thought my sensor was reading low and bought a new unit which when installed read 1.5 lower than the old one, so which was right? Well i called Davis and asked them the same question and was told to send the new one back for calibration. When i got it back it read .1 higher than the old one so the new out of the box sensor was wrong. You need to keep the sensor clean for sure, i use vodka to clean mine lol. By the way i was not aware of the willyweather site but just did a check on it and got a reading only 2 miles from my site. Will be a really good comparison site to keep tabs on my readings. First check was at 9 am and it said a uv of 2 and mine showed 2.3 pretty close. The forecasted high uv for my area yesterday was 11 but i only got to 9 because of the African dust in the air filtering the uv out. Remember there are alot of factors involved in uv readings some filter and some can amplify the readings. My advise is just don't go to crazy with trying to get it dead on it can really be tiring lol

I just want to reiterate that WillyWeather's UV number is a forecast, not a measurement report. The one for my location updates about noon local time, and at that moment has hourly estimates for the current date and the next three days. The next morning it no longer has previous day, but has current and next two. So it adds a day of forecast around noon.

The Willyweather forecast definitely has some attempt to consider the effect of clouds. It also clearly has some other element of non-cloud variation beyond location, time of day, and day of year. I have read that upper atmosphere ozone varies enough to move the result around. I don't know what else shifts UV transparency, but it seems to shift around a LOT more than does solar radiation transparency, at least as logged by my Davis instruments here in Albuquerque, New Mexico.

I have two uv sensors running and they are .1 apart from each other but they weren't always that close. I thought my sensor was reading low and bought a new unit which when installed read 1.5 lower than the old one, so which was right? Well i called Davis and asked them the same question and was told to send the new one back for calibration. When i got it back it read .1 higher than the old one so the new out of the box sensor was wrong. You need to keep the sensor clean for sure, i use vodka to clean mine lol. By the way i was not aware of the willyweather site but just did a check on it and got a reading only 2 miles from my site. Will be a really good comparison site to keep tabs on my readings. First check was at 9 am and it said a uv of 2 and mine showed 2.3 pretty close. The forecasted high uv for my area yesterday was 11 but i only got to 9 because of the African dust in the air filtering the uv out. Remember there are alot of factors involved in uv readings some filter and some can amplify the readings. My advise is just don't go to crazy with trying to get it dead on it can really be tiring lol

HI, i am comparing it to forecast UV levels by Temis and my own wxsim forecast models, it should be around 7 this time of year on a cloudless day, but take today for example, pretty much cloudless apart from the odd very high cloud, my max UV today was 5.3, Temis forecast was 6.7, i am in the UK too, I have literally just check out met office forecast for today and erm, that says 5 dont mind me, must be the sun, we are in a heatwave after all lol

I had mine fail and asked Davis about the UV sensor and they would not disclose what it was. I have since replaced the whole RSS. They UV sensor that they used in that unit is not anything new. Not really that expensive as a single item either.